Can you change the time of day in Pokemon Pearl?

Yes, you can change the time in Pokemon Diamond and Pearl. However, the time is not an in-game function. It is taken from the DS's internal clock. So, to change the time in the game, you must change the time in the DS by accessing the option through the opening menu. The date can also be changed by this method.

You should note, however, that by changing the DS's internal clock, you are setting yourself up for a little trouble in Diamond or Pearl. The game considers your time-change as a method of tampering and so blocks you from accessing daily freebies such as free berries, the Pokemon lottery, etc. It claims that you already obtained them, so you cannot again for another 24 hours. It also effects the Pal Park and the migration of Pokemon from GBA cartridges. You will not be able to migrate Pokemon from any GBA game (with an internal time function) for 24 hours.

No other in-game features are effected by the time change and all saved data remains intact from the last point you saved.